Thin film total internal reflection diffraction grating for single polarization or dual polarization

ABSTRACT

A diffraction grating may include a substrate. The diffraction grating may include an etch stop layer to prevent etching of the substrate. The etch stop layer may be deposited on the substrate. The diffraction grating may include a marker layer to indicate an etch end-point associated with etching of a dielectric layer. The marker layer may be deposited on a portion of the etch stop layer. The diffraction grating may include the dielectric layer to form a grating layer after being etched. The dielectric layer may be deposited on at least the marker layer.

TECHNICAL FIELD

The present disclosure relates to a reflective diffraction grating and,more particularly, to a thin film total internal reflection (TIR)diffraction grating. The present disclosure also relates to a method ofmanufacturing such a thin film TIR diffraction grating.

BACKGROUND

A reflective diffraction grating is used to provide wavelengthdispersion in a wavelength-selective optical device, such as awavelength selective switch (WSS). The reflective diffraction gratingmay be employed (e.g., within a grism) in a double pass configuration,such that an optical path of the WSS results in light passing throughthe reflective diffraction grating twice.

SUMMARY

According to some possible implementations, a diffraction grating mayinclude a substrate; an etch stop layer to prevent etching of thesubstrate, where the etch stop layer may be deposited on the substrate;a marker layer to indicate an etch end-point associated with etching ofa dielectric layer, where the marker layer may be deposited on a portionof the etch stop layer; and the dielectric layer to form a grating layerafter being etched, where the dielectric layer may be deposited on atleast the marker layer.

According to some possible implementations, a diffraction grating, tooperate based on total internal reflection, may include: a substrate; anetch stop layer to prevent etching of the substrate, where the etch stoplayer may be formed on the substrate; a dielectric grating layer on theetch stop layer; and an encapsulation layer to protect the dielectricgrating layer, where the encapsulation layer may be formed on at leastthe dielectric grating layer.

According to some possible implementations, a method of manufacturing adiffraction grating may include: depositing an etch stop layer on asubstrate; depositing a marker layer on a portion of the etch stoplayer; depositing a dielectric layer on the marker layer; and etchingthe dielectric layer to form a grating layer, during etching thedielectric layer, the method may include preventing, by the etch stoplayer, etching of the substrate; and determining, based on etching themarker layer, that etching is to be stopped.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an example prior reflective diffraction grating;

FIG. 2 is a diagram of a first example implementation of a thin filmdiffraction grating designed to operate based on TIR;

FIGS. 3A-3F are diagrams associated with design and performance of thefirst example implementation of FIG. 2;

FIG. 4 is a diagram of a second example implementation of a thin filmdiffraction grating designed to operate based on TIR;

FIGS. 5A-5C are diagrams associated with design and performance of thesecond example implementation of FIG. 4;

FIGS. 6A and 6B are diagrams of third and fourth example implementationsof thin film diffraction gratings designed to operate based on TIR;

FIGS. 7A-7C are diagrams associated with design and performance of thefourth example implementation of FIG. 6B; and

FIG. 8 is a flow chart of an example process for manufacturing a thinfilm TIR diffraction grating described herein.

DETAILED DESCRIPTION

The following detailed description of example implementations refers tothe accompanying drawings. The same reference numbers in differentdrawings may identify the same or similar elements. The implementationsdescribed below are merely examples and are not intended to limit theimplementations to the precise forms disclosed. Instead, theimplementations were selected for description to enable one of ordinaryskill in the art to practice the implementations.

A typical reflective diffraction grating includes a substrate and areflective grating layer. Such a reflective diffraction grating istypically designed to achieve high diffraction efficiency (DE) in the−1^(st) order for a particular polarization of light, such as thetransverse-magnetic (TM) polarization, when the reflective diffractiongrating is in first order Littrow mount. The typical reflectivediffraction grating can be attached to a prism (e.g., with an opticalepoxy) in order to form a grism.

FIG. 1 is a diagram of an example prior reflective diffraction grating100 (herein referred to as prior diffraction grating 100) attached to aprism 120. As shown in FIG. 1, prior diffraction grating 100 includessubstrate 105 and reflective grating layer 110, and is attached to prism120 with optical epoxy 115. As shown in FIG. 1, reflective grating layer110 is embedded in optical epoxy 115 such that a binary grating profileof reflective grating layer 110 (e.g., a series of grooves etched inreflective grating layer 110 to create a series of ridges) is betweensubstrate 105 and prism 120. In some cases, reflective grating layer 110may have a profile different than a binary grating profile, such as asinusoidal grating profile, a triangular grating profile, or the like. Atypical single polarization prior diffraction grating 100 is designedsuch that a DE in a particular order (e.g., the −1^(st) order) for theTM polarization is high (e.g., ≥90%) and a DE in the particular orderfor the transverse-electric (TE) polarization is low (e.g., ≤8%).

Substrate 105 is typically formed of a dielectric material, such asfused silica (SiO₂), or another type of glass. In some cases, reflectivegrating layer 110 is formed of and/or coated with a reflective metallicmaterial, such as gold. However, prior diffraction grating 100 with agold reflective grating layer 110 (herein referred to as gold priordiffraction grating 100) may cause a significant amount of insertionloss. For example, gold prior diffraction grating 100 may have singlepass insertion loss in a range from approximately −0.2 decibels (dB) toapproximately −0.3 dB. Thus, in the typical double pass configuration,gold prior diffraction grating 100 may have insertion loss in a rangefrom approximately −0.4 dB to approximately −0.6 dB.

The insertion loss of gold prior diffraction grating 100 is attributableto at least two factors. One factor that causes this insertion loss isthat the gold of reflective grating layer 110 has a reflectance that isless than 100%. In other words, the gold of reflective grating layer 110does not reflect all light incident upon reflective grating layer 110.This causes a maximum achievable DE for gold prior diffraction grating100 to be less than 100%. For example, for incident light with awavelength of 1550 nanometers (nm), a reflectance of the gold reflectivegrating layer 110 is approximately 97%, which accounts for approximately−0.13 dB of insertion loss.

Another factor that causes the insertion loss associated with gold priordiffraction grating 100 is non-perfect −1^(st) order blazing caused bythe binary grating profile of reflective grating layer 110 upon whichlight is incident (i.e., the non-planar surface of reflective gratinglayer 110 formed by the grooves and ridges). For example, for light witha wavelength of 1550 nm, the binary grating profile of reflectivegrating layer 110 causes non-perfect −1^(st) order blazing that resultsin insertion loss of approximately −0.12 dB or more. Thus, based onthese two factors, a total single pass insertion loss of gold priordiffraction grating 100 may be in a range from approximately −0.25 dB to−0.31 dB, meaning that a DE in the −1^(st) order for the TM polarizationmay range from approximately 93.2% to 94.4%.

One manner in which to improve the DE of prior diffraction grating 100is to improve upon the reflectance of the metallic material (e.g., gold)of reflective grating layer 110. Thus, in some cases, reflective gratinglayer 110 is formed using one or more reflective dielectric thin filmlayers (e.g., rather than the metallic layer). However, while relativelyfew (e.g., fewer than five) dielectric thin film layers can achievenear-perfect blazing (e.g., a DE of approximately 100%) in the −1^(st)order for the TE polarization, a substantially larger number ofreflective dielectric thin film layers (e.g., more than 30) are neededin order to achieve high DE (e.g., greater than 94%) in the −1^(st)order for the TM polarization when the light has a high angle ofincidence (e.g., when prior diffraction grating 100 is in first orderLittrow mount, as in the typical case). As such, manufacture of priordiffraction grating 100 is expensive, time consuming, and/or complexwhen using reflective dielectric thin film layers to achieve high DE inthe −1^(st) order for the TM polarization.

Furthermore, the DE associated with reflective dielectric thin filmlayers in prior diffraction grating 100 may have a significant roll-off(e.g., a sharp decrease in DE) as a wavelength of light deviates from adesign wavelength (e.g., a wavelength near a center of the conventionalband (C band), a wavelength near a center of the long wavelength band (Lband), or the like). Moreover, and unlike a metallic (e.g., gold)reflective grating layer 110, reflective dielectric thin film layer 110permits propagation of at least two transmitted orders into substrate105. This may result in additional insertion loss (e.g., when lightleaks in the transmitted orders) and, thus, may further reduce the DE ofprior diffraction grating 100. Such transmitted orders are illustratedby the “0^(th) Transmitted” and “−1^(st) Transmitted” dashed lines inFIG. 1.

Implementations described herein provide various implementations of thinfilm dielectric reflective diffraction gratings that operate based ontotal internal reflection (TIR) (herein referred to as thin film TIRdiffraction gratings). The thin film TIR diffraction gratings, describedherein, include a small number (e.g., one, two) of reflective dielectricthin film layers, while still achieving high DE (e.g., greater than 94%)in the −1^(st) order for the TM polarization and/or the TE polarization.Furthermore, the thin film TIR diffraction gratings, described herein,prevent propagation of transmitted orders, thereby preventing insertionloss due to light leaking in such transmitted orders. In someimplementations, the thin film TIR diffraction grating may be designedto achieve high DE for a single polarization (e.g., the TM polarizationor the TE polarization), or dual polarizations of light (e.g., the TMpolarization and the TE polarization), as described elsewhere herein.

In some implementations, manufacturability and/or reliability of thethin film TIR diffraction grating may be improved by including an etchstop layer associated with protecting a substrate during etching, amarker layer associated with signaling or indicating an etch end-pointduring etching, and/or an encapsulation layer associated with protectingthe thin film grating layer of the thin film TIR diffraction grating(e.g., during an optical bonding process, during shipping, duringcleaning, or the like), as described below.

FIG. 2 is a diagram of an example implementation of a thin filmdiffraction grating 200 designed to operate based on TIR. As shown inFIG. 2, thin film TIR diffraction grating 200 may include a substrate205, an etch stop layer 210, and a thin film grating layer 215. Asshown, thin film TIR diffraction grating 200 may be attached to opticalelement 220 (e.g., in order to form a grism when optical element 220 isa prism). In some implementations, optical element 220 may have atriangular shape (e.g., optical element 220 may be a prism) or anon-triangular shape (e.g., optical element 220 may be a multi-surfacetrapezoid, a sphere, or the like).

Substrate 205 includes a layer on which additional layers of thin filmTIR diffraction grating 200 may be deposited. In some implementations,substrate 205 may be formed of a dielectric material, such as fusedsilica, or another type of glass.

Etch stop layer 210 includes a layer formed of a material that isresistant to etching. For example, etch stop layer 210 may include alayer formed of aluminum oxide (Al₂O₃) that is resistant to reactive ionetching (RIE). In this case, etch stop layer 210 may ensure that gases,associated with the RIE process, do not penetrate substrate 205 in orderto prevent substrate 205 from being etched (which, if allowed, maynegatively affect a DE of thin film TIR diffraction grating 200). Asshown, etch stop layer 210 may be disposed between substrate 205 andthin film grating layer 215.

Thin film grating layer 215 (sometimes referred to as a dielectricgrating layer) is a reflective dielectric grating layer that diffractsincident light. As shown, thin film grating layer 215 may have a binarygrating profile comprising ridges separated by grooves, where thegrooves may be formed in a layer of dielectric material in order to formthin film grating layer 215 by etching, as described below. In someimplementations, the ridges may be rectangular in cross-section.Additionally, or alternatively, the ridges may be trapezoidal incross-section, or take on another shape. In some implementations, topsof the ridges of thin film grating layer 215 are substantially parallelto a top surface of the substrate 205, and sidewalls of the ridges aresubstantially perpendicular to the top surface of the substrate 205(e.g., when the ridges are rectangular in cross-section). In someimplementations, the grating profile may be sinusoidal, triangular,trapezoidal, or take on another periodic shape. A binary step may be apreferred grating profile because it is simple to manufacture byphotolithographic etching.

In some implementations, thin film grating layer 215 may be formed froma dielectric layer that includes a small number (e.g., one or two)layers of a dielectric material with an index of refraction (n) that ishigher than an index of refraction of air (n=1), such as silicon (Si,n=3.60), tantala (Ta₂O₅, n=2.10), silica (SiO₂, n=1.45), or the like. Insome implementations, the dielectric material from which thin filmgrating layer 215 is formed may be selected or identified based on adesired DE, as described below.

The arrangement of layers of thin film TIR diffraction grating 200allows thin film TIR diffraction grating 200 to operate based on TIR forlight that is incident beyond a critical angle (e.g., as shown in FIG.2). Here, light that is incident beyond the critical angle is notrefracted, but is totally internally reflected (e.g., in the specularlyreflected (0^(th)) order, in the ±1^(st) reflected diffraction orderthat is present when thin film grating layer 215 is attached to thebottom of etch stop layer 210), with 100% reflectance being possible inboth the TM polarization and the TE polarization. The arrangement of thelayers shown in FIG. 2 permits such TIR operation by creating a TIRinterface (e.g., a flat, planar grating/air interface, rather than anon-planar surface as with prior diffraction grating 100) at a lowersurface of etch stop layer 210. TIR operation of thin film TIRdiffraction grating 200 is possible when a pitch of thin film TIRdiffraction grating 200 is less than or equal to half of a designwavelength. The grating pitch is a distance from an edge of a groove ofthin film grating layer 215 to a corresponding edge of an adjacentgroove. A groove width is a distance between edges of a groove. Anormalized groove width is a portion (e.g., a percentage) of the gratingpitch that is a groove (i.e., a portion of the pitch that is not aridge). A grating height is a depth of a groove (i.e., a height of aridge).

Substrate transmitted orders are eliminated due to the TIR operation ofthin film TIR diffraction grating 200 and the arrangement of the layersof thin film TIR diffraction grating 200 (e.g., since thin film gratinglayer 215 is positioned on an outer surface of substrate 205, ratherthan between optical element 220 and substrate 205). Further, thin filmgrating layer 215 need not be embedded in epoxy between optical element220 and substrate 205. Without a need to embed thin film grating layer215 in epoxy (e.g., in order to attach thin film TIR diffraction grating200 to optical element 220), bending or warping of thin film gratinglayer 215 caused by use of such epoxy (e.g., due to expansion orcontraction of the epoxy) is eliminated.

The heights, widths, and thicknesses of layers shown in FIG. 2 areprovided as examples, and are exaggerated for illustrative purposes.Further, thin film TIR diffraction grating 200 may include additionallayers, fewer layers, different layers, or differently arranged layersthan those shown in FIG. 2.

FIGS. 3A-3F are diagrams associated with design and performance of thethin film TIR diffraction grating 200 of FIG. 2. FIG. 3A is a diagram300 of an example graph showing contour areas needed to achieve variousminimum DEs in the −1^(st) order for the TM polarization for thin filmTIR diffraction grating 200 that includes thin film grating layer 215formed of one of silica, tantala, and silicon. Other materials havingsufficiently high refractive indices for the desired wavelengths oflight are also possible. As noted, FIG. 3A corresponds to a thin filmgrating layer 215 with a pitch having 1624 lines (i.e., repeatingpatterns such as a binary ridge and gap) per millimeter (lines/mm), andarranged in Littrow mount. The contour area may refer to a width of agroove of thin film grating layer 215 times a depth of the groove (i.e.,a height of a ridge of thin film grating layer 215).

As shown in FIG. 3A, the size of contour areas are generally larger forthin film grating layers 215 formed of silicon or tantala, rather thansilica. In other words, manufacturability of thin film grating layer 215formed of silicon or tantala may be increased as compared to that ofthin film grating layer 215 formed of silica (e.g., since less fineetching is needed). Thus, in some cases, silicon or tantala may beselected for thin film grating layer 215 in order to increasemanufacturability of thin film grating layer 215. As illustrated in FIG.3A, thin film grating layer 215 should not be formed of silica when thedesired DE in the −1^(st) order for the TM polarization is greater thanapproximately 95%.

As further shown, the size of the contour areas generally decreases(e.g., for silicon, tantala, and silica) as the required minimum DEincreases. In other words, as the desired minimum DE increases, fineretching is needed. For example, when thin film grating layer 215 isformed of silicon, a contour area with a size of approximately 0.055square microns is needed in order to achieve 90% DE in the −1^(st) orderfor the TM polarization. Conversely, when thin film grating layer 215 isformed of silicon, a contour area with a size of approximately 0.005square microns is needed in order to achieve 99% DE in the −1^(st) orderfor the TM polarization. As such, manufacturability of the thin filmgrating layer 215 may generally decrease as the desired DE increases.

Nonetheless, as shown by FIG. 3A, silica, silicon, or tantala may beselected for formation of thin film grating layer 215 in order toachieve a DE from approximately 94% to approximately 95% (with a silicathin film grating layer 215 being the most difficult to manufacture dueto a need for smaller contour areas). Silicon or tantala may be selectedfor formation of thin film grating layer 215 in order to achieve a DEgreater than approximately 95%. Notably, a contour area of a siliconthin film grating layer 215 that is needed to achieve 98% DE or 99% DEis larger than a contour area of a tantala thin film grating layer 215that is needed to achieve 98% DE or 99% DE. Thus, in some cases, siliconmay be selected for formation of thin film grating layer 215 rather thantantala (e.g., since the larger contour areas of silicon thin filmgrating layer 215 are more easily manufactured). However, whileimplementations described below describe thin film grating layer 215 asbeing formed of silicon, in some implementations, thin film gratinglayer 215 may be formed of silica, tantala, or another suitablematerial.

As noted above, FIG. 3A is provided as an example associated withachieving various DEs in the −1^(st) order for the TM polarization for athin film grating layer 215 formed of silica, tantala, and silicon,where thin film grating layer 215 includes 1624 lines/mm, and isarranged in Littrow mount. In practice, thin film grating layer 215 maybe formed of a different material, different pitch (may have additionalor fewer number of lines/mm), may be designed for high DE in a differentorder (e.g., the 0^(th) order), or may otherwise be differentlyconfigured than as described above with regard to FIG. 3A.

FIG. 3B is a diagram of an example thin film TIR diffraction grating 200with particular parameters for a thin film grating layer 215 formed ofsilica, including 1624 lines/mm and an etch stop layer 210 with athickness of 0.05 microns.

As shown in FIG. 3B, example thin film TIR diffraction grating 200 mayinclude a silicon thin film grating layer 215 with a normalized groovewidth of 0.4 (i.e., 40% of a grating pitch of example thin film TIRdiffraction grating 200 is a groove) and a grating height of 0.88microns. As further shown in FIG. 3B, thin film TIR diffraction grating200 may include etch stop layer 210 with a thickness of 0.05 microns.

In some implementations, thin film TIR diffraction grating 200 may bedesigned based on a design space associated with identifying parametersof thin film grating layer 215 in order to achieve high DE in the−1^(st) order for the TM polarization, and low DE in the −1^(st) orderfor the TE polarization, as described below with regard to FIG. 3C. TheDE in the −1^(st) order for the TM polarization and the TE polarizationfor a single polarization thin film TIR diffraction grating 200 (e.g.,such as that described in FIG. 3B) may depend on a wavelength ofincident light. In practice, thin film TIR diffraction grating 200 mayinclude a different number of lines/mm, a different normalized groovewidth, and/or etch stop layer 210 with a different thickness.

FIG. 3C is a diagram of an example design space 310 associated withidentifying parameters (e.g., a normalized groove width, a pitch, agroove width, a grating height) of thin film grating layer 215 for asingle polarization thin film TIR diffraction grating 200 or a dualpolarization thin film TIR diffraction grating 200. FIG. 3C illustratesDEs, for both the TM polarization and the TE polarization, in the−1^(st) order for a thin film grating layer 215 formed of silicon.Further, example design space 310 corresponds to a wavelength within theC band that results in worst-case polarization design spaces (i.e.,smallest polarization design spaces), as described below. It isunderstood that all other wavelengths in the C band would have designspaces overlapping in the same areas and at least the same size as, ifnot larger than, the design spaces illustrated in FIG. 3C.

As shown in FIG. 3C, the vertical axis of example design space 310corresponds to a range of grating heights (e.g., a height of a ridge, adepth of a groove) of thin film grating layer 215 from 0.0 microns to1.0 micron. As further shown, the horizontal axis of example designspace 310 corresponds to a range of groove widths (e.g., air groovewidth (AGW)) of thin film grating layer 215 that are normalized to apitch of thin film grating layer 215 (herein referred to as normalizedgroove widths). In other words, the normalized groove widths correspondto percentage of the pitch of thin film grating layer 215 that is agroove (e.g., rather than a ridge).

As shown by the legend in the right portion of FIG. 3C, theblack-to-light gray gradient of example design space 310 represents DEs(e.g., from 0% to 100%) in to the −1^(st) order for the TE polarizationwithin the grating height range and normalized groove width rangeidentified above. As shown, the DE in the −1^(st) order for the TEpolarization varies across example design space 310. For example, for anormalized groove width of 0.6, and a grating height of 0.1 microns, theDE in the −1^(st) order for the TE polarization is approximately 100%.Similarly, for a normalized groove width of 0.5, and a grating height of0.1 microns, the DE in the −1^(st) order for the TE polarization isapproximately 50%. Further, for a normalized groove width of 0.2, and agrating height of 0.1 microns, the DE in the −1^(st) order for the TEpolarization is approximately 0%.

The transparent black areas (surrounded by white dotted lines in FIG.3C) represent areas of example design space 310 where DEs in the −1^(st)order for the TM polarization are greater than or equal to 90%. Forexample, for a normalized groove width of 0.3, and a grating height of0.35 microns, the DE in the −1^(st) order for the TM polarization isgreater than or equal to 90%.

As noted above, FIG. 3C is provided as an example design space 310 forDEs in the −1^(st) order for a thin film grating layer 215 formed ofsilicon, and corresponds to areas of different DE for TE and TMpolarizations of a wavelength within the C band that results inworst-case polarization design spaces. The example design space 310includes areas of overlap between high DE for the TM polarization andlow DE for the TE polarization (e.g., that may be used for designing asingle polarization thin film TIR diffraction grating 200) or areas ofoverlap between high DE for the TM polarization and high DE for the TEpolarization (e.g., that may be used for designing a dual polarizationthin film TIR diffraction grating 200). Other design spaces (e.g., withwider ranges, smaller ranges, and/or different ranges of grating heightand/or normalized groove width) exist for other wavelengths (e.g.,within the C band, within the L band, or the like) and/or for thin filmgrating layers 215 formed of other materials. In other words, exampledesign space 310 is a single example of a possible design space.

In some implementations, the parameters of thin film grating layer 215may be identified using example design space 310 in order to design thinfilm grating layer 215 to achieve a desired DE associated with one orboth polarizations of light. For example, in a case where thin filmgrating layer 215 is to achieve high (e.g., greater than or equal to94%) DE in the −1^(st) order for the TM polarization, and low (e.g.,less than 10%, approximately 0%) DE in the −1^(st) order for the TEpolarization (i.e., when thin film TIR diffraction grating 200 isdesigned for a single polarization), parameters of thin film gratinglayer 215 may be identified based on approximately the area labeled“single polarization design space” in FIG. 3C. Within the singlepolarization design space, the DE in the −1^(st) order for the TMpolarization is high, while DE in the −1^(st) order for the TEpolarization is low.

As a particular example, a silicon thin film grating layer 215 with anormalized groove width of 0.4 and a grating height of 0.88 microns(similar to that described above with regard to FIG. 3B, and identifiedby a point marked as “x_(s)” within the single polarization designspace), may achieve high DE in the −1^(st) order for the TMpolarization, and low DE in the −1^(st) order for the TE polarization.

FIG. 3D is a diagram of an example graph 320 that shows DEs in the−1^(st) order for the TM polarization and DEs in the −1^(st) order forthe TE polarization for incident light with wavelengths that range from1500 nm to 1600 nm (i.e., across approximately the C band).

As shown in FIG. 3D (e.g., by the line identified as TM R1, and usingthe corresponding left vertical axis), the DEs in the −1^(st) order forthe TM polarization ranges from approximately 96.8% (e.g., at 1500 nmand 1600 nm) to approximately 99.9% (e.g., at approximately 1550 nm). Asfurther shown (e.g., by the line identified as TE R1, and using thecorresponding right vertical axis), the DEs in the −1^(st) order for theTE polarization range from approximately 0.4% (e.g., at 1500 nm) toapproximately 2.1% (e.g., at 1600 nm). Thus, as illustrated in FIG. 3D,a DE in the −1^(st) order for the TM polarization exceedingapproximately 99% can be readily achieved in the C band. This may allowinsertion loss to be improved by approximately 0.4 dB to approximately0.6 dB (e.g., as compared to prior diffraction grating 100). As alsoillustrated, a DE in the −1^(st) order for the TE polarization that isless than approximately 1% can be achieved in the C band, whichcorresponds to an improvement of approximately 5% as compared to priordiffraction grating 100.

Returning to FIG. 3C, in a case where thin film grating layer 215 is toachieve high (e.g., greater than or equal to 94%) DE in the −1^(st)order for the TM polarization, and high DE in the −1^(st) order for theTE polarization (i.e., when thin film TIR diffraction grating 200 isdesigned for dual polarization), parameters of thin film grating layer215 may be identified from approximately the area labeled “dualpolarization design space” in FIG. 3C. Within the dual polarizationdesign space, DE in the −1^(st) order for the TM polarization is high,and DE in the −1^(st) for the TE polarization is high. In a case wherethin film TIR diffraction grating 200 is to minimize polarizationdependent loss (PDL), thin film grating layer 215 may be designed fordual polarization blazing (e.g., since high DE is achieved for bothpolarizations, losses that depend on polarization are reduced).

As a particular example, a thin film grating layer 215 with a normalizedgroove width of 0.7 (e.g. 70% of pitch) and a grating height of 0.38microns (identified by a point marked as “x_(d)” within the dualpolarization design space), may achieve high DE in the −1^(st) order forthe TM polarization, and high DE in the −1^(st) order for the TEpolarization. FIG. 3E is a diagram of an example thin film TIRdiffraction grating 200 with these parameters for a thin film gratinglayer 215 with 1624 lines/mm and an etch stop layer 210 with a thicknessof 0.05 microns. In practice, thin film TIR diffraction grating 200 mayinclude a different number of lines/mm and/or etch stop layer 210 with adifferent thickness.

The DE in the −1^(st) order for the TM polarization and the TEpolarization for a dual polarization thin film TIR diffraction grating200 (e.g., such as that described in FIG. 3E) may depend on a wavelengthof incident light. FIG. 3F is a diagram of an example graph 330 thatshows DEs in the −1^(st) order for the TM polarization, DEs in the−1^(st) order for the TE polarization, and an average DE in the −1^(st)order (i.e., an average of the DE for the TM polarization and the TEpolarization, shown by the line identified as “Av. R1”), for incidentlight with a wavelength in a range from 1500 nm to 1600 nm. FIG. 3F alsoshows DEs in the 0^(th) order for both the TM and TE polarizations(shown by the lines identified as “TM R0” and “TE R0”, respectively).

As shown in FIG. 3F (e.g., by the line identified as “TM R1”), the DE inthe −1^(st) order for the TM polarization ranges from approximately 88%(e.g., at 1600 nm) to approximately 100% (e.g., at approximately 1520nm). As further shown (e.g., by the line identified as “TE R1”), the DEin the −1^(st) order for the TE polarization ranges from approximately95.0% (e.g., at 1500 nm) to approximately 99.0% (e.g., at approximately1560 nm). As such, high DE in the −1^(st) order for both the TEpolarization may be achieved, with relatively low DE in the 0^(th) orderfor both the TM and TE polarizations (e.g., less than approximately 12%and 5% for the TM polarization and the TE polarization, respectively).

Thus, as illustrated in FIG. 3F, a DE in the −1^(st) order exceeding 95%can be readily achieved for both the TM polarization and the TEpolarization in the C band. This corresponds to a worst-case insertionloss of approximately −0.14 dB, and a worst-case PDL of approximately0.16 dB.

Notably, FIGS. 3A-3F are provided merely as examples, and other examplesare possible that may differ from those described in association withFIGS. 3A-3F. For example, thin film grating layer 215 may includeadditional or fewer lines/mm, may be formed of a different material, orthe like. As another example, thin film TIR diffraction grating 200 mayinclude an etch stop layer 210 with a different thickness, may includeadditional and/or different layers (e.g., marker layer 225,encapsulation layer 230, as described below), may be designed for highDE in a different order (e.g., the 0^(th) order), may be designed foruse for light in a larger range of wavelengths, a smaller range ofwavelength, or a different range of wavelengths (e.g., the L band), orthe like. In other words, FIGS. 3A-3F are merely examples associatedwith possible thin film TIR diffraction gratings that operate based onTIR while achieving high DE for the TM polarization and/or the TEpolarization.

FIG. 4 is a diagram of a second example implementation of a thin filmdiffraction grating 235 designed to operate based on TIR. As shown inFIG. 4, thin film TIR diffraction grating 235 may include substrate 205,etch stop layer 210, thin film grating layer 215, and a marker layer225. As shown, thin film TIR diffraction grating 235 may be attached tooptical element 220 (e.g., in order to form a grism). As illustrated inFIG. 4, the diffraction grating 235 is attached to optical element 220by the side of the substrate 205 opposite to the thin film grating layer215 (e.g., the bottom of the substrate 205).

As shown, thin film TIR diffraction grating 235 may have a structurethat is similar to thin film TIR diffraction grating 200 (e.g., asimilar arrangement of substrate 205, etch stop layer 210, and thin filmgrating layer 215). In addition to these layers, thin film TIRdiffraction grating 235 may include marker layer 225 disposed betweenetch stop layer 210 and thin film grating layer 215.

Marker layer 225 includes a layer associated with signaling, indicating,and/or identifying an etch end-point (e.g., a point at which etchingshould stop) during etching of the thin film dielectric material fromwhich thin film grating layer 215 is formed. In some implementations,marker layer 225 may be formed of an etchable material, such as silicon,silicon nitride (Si₃N₄), tantala, or the like. In some implementations,marker layer 225 may have a thickness that is less than approximately0.1 micron, such as 50 nm.

In some implementations, manufacturability of thin film TIR diffractiongrating 235 may be improved by marker layer 225 (e.g., as compared tomanufacturability of thin film TIR diffraction grating 200). Forexample, grooves may be etched (e.g., toward substrate 205) in the thinfilm dielectric layer in order to form thin film grating layer 215. In atypical RIE etching process (e.g., one that uses fluorine basedchemistry), when the etching reaches substrate 205, reactants producedby the etching (e.g., silicon fluoride (SiF), C—N, tantalum fluoride(TaF) are detected by a mass spectrometer associated with an etchchamber. Here, when the mass spectrometer detects an increase, a spike,a peak, or the like, in an amount of the reactants being produced (e.g.,due to the etching reaching and penetrating substrate 205), the massspectrometer causes etching to stop. In other words, the massspectrometer detects the etch end-point based on the reactants producedwhen the etching penetrates substrate 205, rather than by monitoring anamount of time that etching is performed.

However, since etch stop layer 210 layer is resistant to etching, asdescribed above, the reactants needed to detect the etch end-point arenot produced (e.g., since the etching does not penetrate substrate 205).In such a case, time-based etching may be used. However, precise timing,through etch rate calibration, may be employed in order to determine theend-point. Due to natural variations in an etch rate of a particularetching chamber, and variations in etch rates between different etchingchambers, such timing is difficult to ensure for different etches (e.g.,different etches using a same chamber, different etches using differentchambers). Thus, over-etching of thin film grating layer 215 in alateral direction (e.g., into sidewalls of the ridges) and/or underetching of thin film grating layer 215 may result during a giventime-based etch. Such etch variations will impact an overall etchprocess yield since grating profile tolerances cannot be reliablyachieved. In other words, the etching process may not be repeatableusing the time-based approach.

Marker layer 225 may improve manufacturability of thin film TIRdiffraction grating 235 by permitting the reactant detection-basedetching technique, described above, to be used for etching thedielectric layer, from which thin film grating layer 215 is formed, whenthin film TIR diffraction grating 235 includes etch stop layer 210. Forexample, marker layer 225 may be formed of silica or silicon nitride.Here, when the etching reaches and penetrates marker layer 225, thereactant (e.g., silicon fluoride) may be produced due to the penetrationof marker layer 225. In other words, the etching of marker layer 225 maycause marker layer 225 to signal or indicate the etch end-point (byproducing the detectable reactant). As such, a mass spectrometer may becapable of detecting an increase, a spike, or a peak in an amount of thereactant, and may cause the etching to stop, accordingly. This mayimprove manufacturability of thin film TIR diffraction grating 235 byensuring that grating profile tolerances can be reliably achieved in arepeatable manner.

The heights, widths, and thicknesses of layers shown in FIG. 4 areprovided as examples, and are exaggerated for illustrative purposes. Inpractice, thin film TIR diffraction grating 235 may include additionallayers, fewer layers, different layers, or differently arranged layersthan those shown in FIG. 4.

In some implementations, a DE of thin film TIR diffraction grating 235may not be significantly impacted due to the inclusion of marker layer225 in thin film TIR diffraction grating 235. FIG. 5A is a diagram of anexample graph 500 that shows the DE in the −1^(st) order for the TMpolarization for thin film TIR diffraction grating 235 including markerlayer 225, formed of silica and silicon nitride, with thickness thatranges from 0.001 microns to approximately 0.8 microns. As noted, FIG.5A corresponds to a silicon thin film grating layer 215 with a pitchhaving 1624 lines/mm, and arranged in Littrow mount.

As shown in FIG. 5A (by the line identified as “minDE TM SiO2 Marker”),when marker layer 225 is formed of silica, the DE in the −1^(st) orderfor the TM polarization is greater than 98% when the thickness of markerlayer 225 is less than or equal to approximately 0.065 microns, and isgreater than 99% when the thickness of marker layer 225 is less than orequal to approximately 0.025 microns.

Similarly (as shown by the line identified as “minDE TM Si3N4 Marker”),when marker layer 225 is formed of silicon nitride, the DE in the−1^(st) order for the TM polarization is greater than 98% when thethickness of marker layer 225 is less than or equal to approximately0.81 microns, and is greater than or equal to 99% when the thickness ofmarker layer 225 is less than or equal to approximately 0.041 microns.

Notably, a roll-off in the DE that results from a silicon nitride markerlayer 225 is less dramatic than that of a silica marker layer 225 (e.g.,when the thickness of marker layer 225 exceeds 0.1 micron). However, asillustrated, when marker layer 225 is less than or equal toapproximately 0.05 microns (50 nm), the DE in the −1^(st) order is notsignificantly affected, regardless of whether marker layer 225 is formedof silica or silicon nitride.

Thus, in some implementations, marker layer 225 may be deposited in agrating area of thin film TIR diffraction grating 235. In other words,during manufacture of thin film TIR diffraction grating 235, markerlayer 225 may be deposited within a chip boundary (i.e., in an on-chiparea) of substrate 205 (e.g., a wafer) on which thin film TIRdiffraction grating 235 is formed. Here, portions of marker layer 225remain in the ridges of thin film TIR diffraction grating 235 and aredisposed between thin film grating layer 215 and etch stop layer 210(e.g., as shown in FIG. 4).

Additionally, or alternatively, marker layer 225 may be deposited in anoff-chip area (e.g., a process control monitoring (PCM) region, an areaoutside of the chip boundary) of substrate 205 on which thin film TIRdiffraction grating 235 is formed (e.g., rather than being deposited inthe on-chip area, or in addition to being deposited in the on-chiparea).

FIG. 5B is a diagram of an example wafer 510 that shows marker layer 225as being deposited in an off-chip area of a wafer on which multiple thinfilm TIR diffraction gratings 235 are formed. As shown in FIG. 5B, onexample wafer 510, marker layer 225 may be deposited in off-chip areasadjacent to ends of each chip boundary. As noted in FIG. 5B, in thisexample, marker layer 225 is not deposited within chip boundaries (i.e.,is not deposited in on-chip areas) of wafer 510. Here, wafer 510 may bemasked such that etching takes place within grating areas (e.g.,identified by white areas with vertical lines) inside the chipboundaries, and within the off-chip areas where marker layer 225 ispresent (e.g., identified by gray areas with vertical lines). Here, whenthe etching penetrates marker layer 225 in the off-chip areas, reactantsdetectable by a mass spectrometer may be produced (i.e., marker layer225 may signal or indicate the etch end-point), and etching may bestopped, as described above. In this example, the resulting thin filmTIR diffraction grating will not include marker layer 225, and may besimilar to thin film TIR diffraction grating 200 described above withregard to FIG. 2.

FIG. 5C is a partial cross section of example wafer 510 that includesmarker layer 225 in off-chip areas, and does not include marker layer225 in on-chip areas. As shown, marker layer 225 is present in anoff-chip area (e.g., outside of a chip boundary), and is not present inan on-chip area (e.g., within the chip boundary). As further shown, insome implementations, wafer 510 may include an un-etched region betweenthe off-chip area with marker layer 225 and the on-chip area withoutmarker layer 225.

Notably, FIGS. 5A-5C are provided merely as examples, and other examplesare possible that may differ from those described in association withFIGS. 5A-5C. For example, thin film grating layer 215 may includeadditional or fewer lines/mm, may be formed of a different material, orthe like. As another example, thin film TIR diffraction grating 235 mayinclude an etch stop layer 210 and/or marker layer 225 with a differentthickness, may include additional and/or different layers (e.g.,encapsulation layer 230, as described below), may be designed for highDE in a different order (e.g., the −2^(nd) order, the −3^(rd) order),may be designed for use with light in a larger range of wavelengths, asmaller range of wavelength, a different range of wavelengths, or thelike. As an additional example, marker layer 225 may be included in anon-chip area that is off of thin film TIR diffraction gratings 235. Inother words, FIGS. 5A-5C are merely examples associated with possiblethin film TIR diffraction gratings that operate based on TIR whileachieving high DE for the TM polarization and/or the TE polarization,and are manufactured using a marker layer associated with indicating anetch end-point.

FIGS. 6A and 6B are diagrams of example implementations of thin filmdiffraction gratings 240 and 245, respectively, designed to operatebased on TIR. As shown in FIG. 6A, thin film TIR diffraction grating 240includes substrate 205, etch stop layer 210, thin film grating layer215, and an encapsulation layer 230. As shown, thin film TIR diffractiongrating 240 may be attached to optical element 220 (e.g., in order toform a grism).

As shown, thin film TIR diffraction grating 240 may have a structurethat is similar to thin film TIR diffraction grating 200 (e.g., asimilar arrangement of substrate 205, etch stop layer 210, and thin filmgrating layer 215). In addition to these layers, thin film TIRdiffraction grating 240 includes encapsulation layer 230.

Encapsulation layer 230 includes a permanent layer designed to cover,encapsulate, and/or protect thin film grating layer 215 of thin film TIRdiffraction grating 240. In some implementations, encapsulation layer230 may be formed of a hard, scratch resistant dielectric material, suchas fused silica, glass (e.g., spin-on glass, atomic layer deposition(ALD) deposited SiO₂), or the like. As shown in FIG. 6A, a thickness ofencapsulation layer 230 may be greater than a thickness of thin filmgrating layer 215 (e.g., a height of a ridge of thin film grating layer215).

In some implementations, encapsulation layer 230 may prevent thin filmgrating layer 215 from being touched, damaged, scratched, contaminated,or otherwise contacted, during, for example, grism assembly (e.g.,during bonding step, a polishing step, when integrating the grating orgrism into an opto-mechanics/optical bench, or the like), shipping,handling by a person, or the like. Here, use of encapsulation layer 230eliminates a need to apply a removable protective material (e.g., CanadaBalsam, a protective paint) to thin film grating layer 215, which may beadvantageous since the removable protective material may not fullyprotect thin film grating layer 215, may be difficult to remove, or maycause damage during removal/cleaning, or the like.

Moreover, encapsulation layer 230 may be polished, without damaging thinfilm grating layer 215, in order to flatten or planarize anencapsulation surface (e.g., the lowermost surface of thin film TIRdiffraction grating 240 as shown in FIG. 6A) of thin film TIRdiffraction grating 240. For example, due to the use of encapsulationlayer 230, the TIR interface of thin film TIR diffraction grating 240 islocated at the encapsulation surface of encapsulation layer 230, asshown in FIG. 6A (e.g., rather than a bottom of etch stop layer 210 aswith thin film TIR diffraction gratings 200 and 235). Thus, theencapsulation surface of thin film TIR diffraction grating 240 may beflattened or planarized in order to prevent a reduction in the DE thatwould result from a non-planar or rough TIR interface. Here, theencapsulation surface of thin film TIR diffraction grating 240 may bemanipulated, touched, polished, and cleaned, without risk of damage tothin film grating layer 215 that would negatively affect the DE of thinfilm TIR diffraction grating 240. In some implementations, sides of thegrating chip may be polished in order to coincide with sides of prism 22to form an input surface for an optical beam.

Furthermore, encapsulation layer 230 allows removable protectivematerial to be applied to the now planar encapsulation surface that canbe readily removed (e.g., with a typical swab and solvent cleaningprocess).

In some implementations, a thin film TIR diffraction grating may includemarker layer 225 and encapsulation layer 230. For example, as shown inFIG. 6B, thin film TIR diffraction grating 245 may include substrate205, etch stop layer 210, thin film grating layer 215, marker layer 225,and encapsulation layer 230. As shown, thin film TIR diffraction grating245 may have a structure that is similar to thin film TIR diffractiongrating 235 (e.g., a similar arrangement of substrate 205, etch stoplayer 210, thin film grating layer 215, and marker layer 225).

The heights, widths, and thicknesses of layers shown in FIGS. 6A and 6Bare provided as examples, and are exaggerated for illustrative purposes.In practice, thin film TIR diffraction grating 240 and/or thin film TIRdiffraction grating 245 may include additional layers, fewer layers,different layers, or differently arranged layers than those shown inFIGS. 6A and 6B.

In some implementations, the DE of thin film TIR diffraction grating 245may depend on, but may not be significantly impacted by, a thickness ofencapsulation layer 230. FIG. 7A is a diagram of thin film TIRdiffraction grating 245 that includes encapsulation layer 230 with athickness that is greater than a thickness of thin film grating layer215. As shown, a difference in thickness between encapsulation layer 230and thin film grating layer 215 is identified as a distance d_(T).

FIG. 7B is a diagram of an example graph 710 that shows DE in the−1^(st) order for the TM polarization for thin film TIR diffractiongrating 245 including encapsulation layer 230 with thickness differences(d_(T)) that range from 0.00 microns to approximately 0.37 microns. Asnoted, FIG. 7B corresponds to a thin film TIR diffraction grating 245that includes a silica encapsulation layer 230, a silicon or silica thinfilm grating layer 215, an aluminum oxide etch stop layer 210, and asilica marker layer 225. Further, FIG. 7B shows the DE in the −1^(st)order for the TM polarization, and corresponds to a wavelength withinthe C band that results in worst-case DE (i.e., the DE may be the sameor higher for other wavelengths in the C band).

As shown in FIG. 7B (by the line identified as “min DE C-Band . . . ”),when d_(T) is 0.0 microns (i.e., when encapsulation layer 230 is a samethickness as thin film grating layer 215), the DE in the −1^(st) orderfor the TM polarization is approximately 98%, which is approximately a1% decrease as compared to a thin film TIR diffraction grating 240without encapsulation layer 230. As shown, for a d_(T) that is between0.0 microns and approximately 0.15 microns, the DE is further decreased(i.e., below 98%). However, as shown, for a d_(T) that is betweenapproximately 0.15 microns and approximately 0.28 microns, the DE isrecovered (i.e., to at least 98%), with a peak DE of approximately 99.5%when d_(T) is approximately equal to 0.25 microns. In other words, thinfilm TIR diffraction grating 245 may have increased DE as compared tothin film TIR diffraction grating 235. As further shown, for a d_(T)that is greater than approximately 0.28 microns, the DE is againdecreased, and there exists a significant DE roll-off.

FIGS. 7A and 7B are provided merely as examples, and other examples arepossible that may differ from those described in association with FIGS.7A and 7B. For example, thin film grating layer 215 may includeadditional or fewer lines/mm, may be formed of a different material, orthe like. As another example, thin film TIR diffraction grating 235 mayinclude an etch stop layer 210, a marker layer 225, and/or anencapsulation layer 230 with a different thickness, may includeadditional and/or different layers, may be designed for high DE in adifferent order (e.g., the −2^(nd) order, the −3^(rd) order), may bedesigned for use with light in a larger range of wavelengths, a smallerrange of wavelength, or a different range of wavelengths, or the like.In other words, FIGS. 7A and 7B are merely examples associated withpossible thin film TIR diffraction gratings that operate based on TIRwhile achieving high DE for the TM polarization and/or the TEpolarization, and include an encapsulation layer associated withprotecting a thin film grating layer.

FIG. 7C is a diagram of an example design space 720 associated withidentifying parameters (e.g., a normalized groove width, a pitch, agroove width, a grating height) of a single polarization thin film TIRdiffraction grating 245 or a dual polarization thin film TIR diffractiongrating 245, where dr is equal to 0.25 microns. FIG. 7C illustrates DEs,for both the TM polarization and the TE polarization, in the −1^(st)order for a thin film grating layer 215 formed of silicon. Further,example design space 720 corresponds to a wavelength within the C bandthat results in worst-case polarization design spaces (i.e., smallestpolarization design spaces).

As shown in FIG. 7C, the vertical axis of example design space 720corresponds to a range of encapsulated grating heights (e.g., a varyingthickness of thin film grating layer 215 plus a constant dr ofapproximately 0.25 microns) from 0.0 microns to 2.0 microns. As furthershown, the horizontal axis of example design space 720 corresponds to arange normalized groove widths from 0.0 to 1.0.

As shown by the legend in the right portion of FIG. 7C (and similar toexample design space 310 described above), the black-to-light graygradient of example design space 720 represents DEs (e.g., from 0% to100%) in the −1^(st) order for the TE polarization within theencapsulated grating height range and normalized groove width rangeidentified above. The transparent black areas (surrounded by whitedotted lines in FIG. 7C) represent areas of example design space 720where DEs in the −1^(st) order for the TM polarization are greater thanor equal to 90%.

In some implementations, the parameters (e.g., a normalized groovewidth, a pitch, a groove width, a grating height including d_(T)) ofthin film TIR diffraction grating 245 may be identified using exampledesign space 720 and/or in order to achieve a desired DE associated withone or both polarizations of light, in a manner similar to thatdescribed above with regard to example design space 310.

As illustrated by example design space 720, a single polarization designspace region occurs for thin film TIR diffraction grating 245 with anencapsulated grating height equal to approximately 1.1 microns, which isapproximately 0.2 microns thicker than the single polarization designspace for thin film TIR diffraction grating 200 associated with exampledesign space 310. Additionally, as shown, a dual polarization designspace also exists for a 1.1 micron encapsulated grating height. Thus, insome implementations, a same grating height may be employed to realize asingle polarization thin film TIR diffraction grating 245 or a dualpolarization thin film TIR diffraction grating 245, which may reducemanufacturing costs. In other words, it is possible to manufacturewafers with the same grating height and then process them differently,masking different groove widths associated with the different designspaces.

As noted above, FIG. 7C is provided as an example design space 720 forDEs in the −1^(st) order for a thin film grating layer 215 formed ofsilicon, and corresponds to a wavelength within the C band that resultsin worst-case polarization design spaces (i.e., areas of overlap betweenhigh DE for the TM polarization and low DE for the TE polarization,areas of overlap between high DE for the TM polarization and high DE forthe TE polarization). Other design spaces (e.g., with wider ranges,smaller ranges, and/or different ranges of grating height and/ornormalized groove width) exist for other wavelengths (e.g., within the Cband, within the L band, or the like) and/or thin film grating layers215 formed of other materials. In other words, example design space 720is a single example of a possible design space.

FIG. 7C is merely an example associated with possible thin film TIRdiffraction gratings that operate based on TIR while achieving high DEfor the TM polarization and/or the TE polarization, and include anencapsulation layer associated with protecting a thin film gratinglayer.

FIG. 8 is a flow chart of an example process 800 for manufacturing thinfilm TIR diffraction grating 245, as described herein. Notably, whileexample process 800 is described in the context of manufacturing thinfilm TIR diffraction grating 245, other thin film TIR diffractiongratings described herein (e.g., thin film TIR diffraction gratings 200,235, or 240) may be manufactured using a similar process (e.g., using asubset of blocks of example process 800).

At block 805, example process 800 may include providing substrate 205(i.e., a wafer) on which thin film TIR diffraction grating 245 is to beformed. At block 810, etch stop layer 210, associated with preventingetching of substrate 205, is deposited on or over substrate 205. Atblock 815, marker layer 225, associated with signaling or indicating anetch end-point during etching, is deposited on or over etch stop layer210. In some implementations, as described above, marker layer 225 maybe deposited within an on-chip area and/or an off-chip area of substrate205. At block 820, a dielectric thin film layer, from which thin filmgrating layer 215 is to be formed, is deposited on or over marker layer225 and/or etch stop layer 210 layer.

At step 825, a photoresist layer is patterned over the dielectric thinfilm layer in order to mask portions of the dielectric thin film layerthat are not to be etched during formation of thin film grating layer215. At block 830, the dielectric thin film layer is etched through thepatterned photoresist layer to form thin film grating layer 215. Here,the etching may proceed until the etching penetrates marker layer 225such that marker layer 225 signals or indicates the etch end-point byproducing reactants for detection by, for example, a mass spectrometer.Upon detecting an increase or peak in the amount of the reactants, themass spectrometer may cause the etching to stop. At block 835, thephotoresist layer is removed.

At block 840, encapsulation layer 230 is deposited on or over thin filmgrating layer 215, exposed portions of etch stop layer 210 and/or markerlayer 225 within grooves of thin film grating layer 215 (e.g., such thatthe grooves are filled with encapsulation layer 230). Encapsulationlayer 230 may be deposited such that a difference between a thickness ofencapsulation layer 230 and a thickness of thin film grating layer 215(e.g., a height of a ridge of thin film grating layer 215) is a desireddistance. In some implementations, encapsulation layer 230 may beplanarized after being deposited. In some implementations, encapsulationlayer may be planarized to reduce the thickness of the encapsulationlayer to approximately 0.25 microns greater than the thickness of thegrating layer. In some implementations, the marker layer is formed oftantala, silica or silicon nitride and has a thickness that is less thanor equal to approximately 50 nanometers. In some implementations, anarea corresponding to the portion of the etch stop layer on which themarker layer is deposited is an off-chip area of the substrate.

In some embodiments, the process 800 may include attaching, bonding orotherwise joining the diffraction grating to a prism by the side of thesubstrate opposite to the thin film grating layer (e.g., the bottom ofthe substrate 205) to form a grism.

Although FIG. 8 shows example blocks of process 800, in someimplementations, process 800 may include additional blocks, fewerblocks, different blocks, or differently arranged blocks than thosedepicted in FIG. 8. Additionally, or alternatively, while exampleprocess 800 describes layers of thin film TIR diffraction grating 245 asbeing deposited, in some implementations, layers of thin film TIRdiffraction grating 245 may be fabricated in another manner, such as bybeing grown, formed, chemically reacted, sprayed, or the like.

Implementations described herein provide various implementations of thinfilm dielectric reflective diffraction gratings that operate based ontotal internal reflection (TIR). The thin film TIR diffraction gratings,described herein, include a small number (e.g., one, two) of reflectivedielectric thin film layers, while still achieving high DE (e.g.,greater than or equal to 94%) in the −1^(st) order for the TMpolarization and/or the TE polarization. Furthermore, the thin film TIRdiffraction gratings, described herein, prevent propagation oftransmitted orders, thereby preventing insertion loss due to lightleaking in such transmitted orders. In some implementations, the thinfilm TIR diffraction grating may be designed to achieve high DE for asingle polarization (e.g., the TM polarization or the TE polarization),or dual polarizations (e.g., the TM polarization and the TEpolarization) of light.

The foregoing disclosure provides illustration and description, but isnot intended to be exhaustive or to limit the implementations to theprecise form disclosed. Modifications and variations are possible inlight of the above disclosure or may be acquired from practice of theimplementations.

Even though particular combinations of features are recited in theclaims and/or disclosed in the specification, these combinations are notintended to limit the disclosure of possible implementations. In fact,many of these features may be combined in ways not specifically recitedin the claims and/or disclosed in the specification. Although eachdependent claim listed below may directly depend on only one claim, thedisclosure of possible implementations includes each dependent claim incombination with every other claim in the claim set.

No element, act, or instruction used herein should be construed ascritical or essential unless explicitly described as such. Also, as usedherein, the articles “a” and “an” are intended to include one or moreitems, and may be used interchangeably with “one or more.” Furthermore,as used herein, the term “set” is intended to include one or more items(e.g., related items, unrelated items, a combination of related items,and unrelated items, etc.), and may be used interchangeably with “one ormore.” Where only one item is intended, the term “one” or similarlanguage is used. Also, as used herein, the terms “has,” “have,”“having,” or the like are intended to be open-ended terms. Further, thephrase “based on” is intended to mean “based, at least in part, on”unless explicitly stated otherwise.

1-20. (canceled)
 21. A diffraction grating, comprising: an etchablelayer that signals an end-point during etching of a dielectric layer,the etchable layer being between the dielectric layer and a substrate;and the dielectric layer that forms a periodic grating layer after beingetched, the dielectric layer being on at least the etchable layer. 22.The diffraction grating of claim 21, further comprising; an etch stoplayer to prevent etching of the substrate, the etch stop layer beingbetween the etchable layer and the substrate.
 23. The diffractiongrating of claim 21, further comprising: a protective layer on at leastthe periodic grating layer.
 24. The diffraction grating of claim 21,where the etchable layer comprises tantala, silica or silicon nitride.25. The diffraction grating of claim 21, where a thickness of theetchable layer is less than approximately 0.1 microns.
 26. Thediffraction grating of claim 21, where the dielectric layer comprisessilicon or tantala.
 27. The diffraction grating of claim 21, where theetchable layer signals the end-point by producing a detectable reactantwhen the etching penetrates the etchable layer.
 28. The diffractiongrating of claim 27, where the detectable reactant comprises siliconfluoride.
 29. A diffraction grating, to operate based on total internalreflection, comprising: a dielectric grating layer between a substrateand a protective layer, the dielectric grating layer being a periodicgrating; and the protective layer to protect the dielectric gratinglayer, the protective layer having a planar surface that spans a widthof the dielectric grating layer.
 30. The diffraction grating of claim29, further comprising: an etchable layer to signal an end-point duringetching of the dielectric grating layer, the etchable layer beingbetween the dielectric grating layer and the substrate.
 31. Thediffraction grating of claim 29, where the protective layer comprisesfused silica or glass.
 32. The diffraction grating of claim 29, where athickness of the protective layer is greater than a thickness of thedielectric grating layer.
 33. The diffraction grating of claim 29, wherea total internal reflection (TIR) interface of the diffraction gratingis at a surface of the protective layer.
 34. The diffraction grating ofclaim 29, where the diffraction grating is included in a grism assembly.35. A method of manufacturing a diffraction grating, the methodcomprising: depositing a signal layer over a portion of a substrate;depositing a dielectric layer on the signal layer; etching thedielectric layer to form a grating layer; etching the signal layer; anddetecting a reactant during the etching of the signal layer, thereactant signaling an etch end-point.
 36. The method of claim 35,further comprising: depositing an encapsulation layer associated withprotecting the grating layer, the encapsulation layer to be deposited onthe grating layer.
 37. The method of claim 36, where a differencebetween a thickness of the encapsulation layer and a thickness of thegrating layer is approximately 0.25 microns.
 38. The method of claim 36,where a total internal reflection (TIR) interface of the diffractiongrating is at a surface of the encapsulation layer.
 39. The method ofclaim 35, where the signal layer has a thickness that is less thanapproximately 0.1 microns.
 40. The method of claim 35, where thereactant comprises silicon fluoride.